1. Field of the Invention
The invention relates to a reciprocating piston machine for use as a prime mover, either as an internal combustion engine or as an external combustion engine, or when supplied with power for use as a fluid pump or compressor.
2. Prior Art
Reciprocating piston machines have been known for many years, a common basic piston arrangement being a piston connected by a straight connecting rod to a crankpin carried on a crankshaft which rotates about a crankshaft axis. As the crankshaft rotates, the piston reciprocates within a cylinder having a longitudinal axis which intersects the crankshaft axis. Whilst this arrangement has many limitations and relative inefficiencies relating to the piston and crankshaft geometry, its mechanical simplicity and reliability has resulted in its almost universal adoption with many variations as the basic major prime mover mechanism. This basic mechanism has also been adopted in many types of reciprocating, positive displacement fluid pumps and compressors.
When this basic mechanism is used in an internal combustion engine, a first limitation relates to the comparatively short dwell of the piston at its top dead center position (TDC) prior to a power-stroke. Force generated by an expanding working gas in the combustion chamber cannot be fully utilized by the time the piston approaches the end of the power-stroke, particularly in relatively high speed two-stroke cycle engines in which the piston uncovers the exhaust port well before total energy in the gas has been utilized.
A second limitation of the common two-stroke engine is that the piston has a relative short dwell at bottom dead center (BDC) and the expanding gases from the power stroke are not fully scavenged from the cylinder. Because the piston dwell at BDC is relatively short, there is little time to induce a fresh charge into the chamber and thus high induction velocities are required which sometimes causes a portion of the fresh charge to be lost through the exhaust port. The two limitations above tend to reduce efficiency of the two-stroke engine. In a four-stroke cycle engine, the relatively short dwell of the piston at TDC similarly does not permit full utilization of available energy from the expanding gases in the combustion chamber.
A third limitation of the conventional piston machine relates to the basic geometry, in particular the line of action and direction of force from the connecting rod as it is applied to the crankshaft during the power stroke. Force from the expanding gas is applied to the crankshaft in a disadvantageous manner for most of the power stroke, thus reducing available output torque. As is well known, increasing throw of the crankpin and thus stroke of the piston can improve torque, but this increase results in higher piston speeds with other disadvantages and thus relatively long-stroke engines are not commonplace.
A fourth limitation of the basic engine relates to the application of the force from the piston to the crankshaft. The modern high compression engine has a relatively large bore and the total high compression forces are transmitted through the connecting rod and crankweb to the crankshaft. These forces require relatively massive components to sustain the high loads, and have correspondingly high inertia which is disadvantageous for reciprocating parts.
A fifth limitation relates to side forces between the piston and cylinder wall arising from the obliquely disposed connecting rod as it follows the piston through its stroke. The side forces cause assymetrical cylinder and ring wear and, other factors being equal, the wear increases as the connecting rod is made shorter. Thus compact or physically small engines with short connecting rods suffer from higher side forces than longer cylinder engines, which tends to aggravate cylinder wear.
As is well known, reciprocating pumps and compressors have corresponding and equivalent disadvantages relating to the piston and crankshaft geometry. As can be seen there are considerable limitations in basic engine design parameters which are inherent in the conventional mechanism.